test(wallet): cover TxBuilder drain and pending-coin filters on regtest

[darioAnongba]

Summary

• add regtest coverage for drain_wallet() with drain_to() producing a single drain output
• add regtest coverage showing exclude_unconfirmed() and exclude_below_confirmations(1) ignore trusted pending coins during coin selection
• note the extra issue test: improve test coverage #22 coverage in CHANGELOG.md

Closes #22

Validation

• cargo fmt --check
• cargo clippy --all-features --all-targets -- -D warnings
• cargo test --all-features
• cd tests/node && corepack yarn lint
• cd tests/node && corepack yarn build (blocked in this environment: missing clang for wasm32-unknown-unknown / wasm-pack build)